Unless you're doing an insane amount of highway driving, 3.73 gears should be the choice for a street driven GT in my opinion.
I hit the highway quite a bit, and have never missed my stock gear ratio....
An h\x-pipe and catback combo should net anywhere from 5-10 rwhp, depending on the combo and the engine.
The head swap to the 99-00 heads is quite a bit cheaper, but requires more parts and minor modification work. The motorsport heads bolt right on and give you a bit more power. No muss, no fuss... but are much more expensive.
The 99-00 heads will also increase your compression a little more than the motorsport pieces, which may be a concern if you go with a blower or nitrous....
(estimate a 30-35 hp increase with either head combo)
As far as the supercharger, if you're looking for boost in the 6 psi range, why not go with a "roots" type blower that is actually part of the intake manifold?
Replace the stock plastic intake with one that has a blower on top...
It gives 6 psi of boost, that gives mucho torque at low rpms.... Press the pedal and go... No Waiting...
(estimate a 55hp increase on a stock engine)
This setup would be sweet on a street driven stang.... a real stoplight warrior...
(Please note that all hp estimates are at the crank. Although the addition of all these parts together would improve the power output at the wheels)
All in all, I'd say you'd be looking at around 300 rwhp with this combo, maybe a bit more...
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"The Yellowjacket"
381.8 rwhp 377.6 rwtq
"Never underestimate the power of stupid people in large groups."